Heat exchanging apparatus and method of manufacture

ABSTRACT

Heat exchange surfaces are formed on a core object, by placing at least a part of a thermally conductive core object within a mold cavity that defines one or more heat exchange surfaces. A heated metal slurry such as, e.g., a magnesium alloy heated to a thixotropic state is injected under a predetermined pressure into the mold cavity. The heated metal slurry is then cooled to form a substantially continuous void free interface between the core object and the slurry when hardened.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention concerns apparatus for effecting heattransfer, and a method of making such apparatus.

[0003] 2. Discussion of the Known Art

[0004] Current trends toward miniaturization of electrical andelectronic devices, have yielded products that need efficient heatdissipation in order to operate properly. This is due to the fact thatsuch products typically consume a relatively large amount of electricalcurrent with respect to their physical size. Cooling techniques such as,e.g., metal heat sinks and fans are used to maintain the operatingtemperatures of electronic components and devices at safe values, sothat they will continue to operate over their expected lifetimes withoutfailure caused by excessive heating. In particular, semiconductor andother solid state devices designed to operate at high power levels aretypically joined to finned, cast aluminum heat sinking structures.Adequate heat dissipation is especially important for electrical powersupplies, radio frequency transmitters, modern desk-top and notebookcomputers, cellular telephones, and most all modern consumer electronicsproducts.

[0005] It is generally known that structures used for transferring heataway from a heat source should have relatively high thermalconductivities (i.e., low thermal resistance), and have exposed surfacesof sufficient area to allow heat conducted from the source to radiateinto a lower temperature environment. Further, the physical interfacebetween a heat sink structure and its associated heat source shouldextend over as large an area as possible with minimal thermalresistance. Use of thermally conductive pastes such as a zinc-oxidesilicone compound at the interface is a common practice. The compoundfills air voids that are created when part of the heat sink structure isjoined against a surface of a component to be cooled. In the absence ofsuch a compound, the air voids act as thermal insulators and reduce theoverall efficiency of the heat sink structure.

[0006] Heat sink configurations in the form of aluminum or copperradiating fins are also arranged on the circumference of heat pipesthrough which a working fluid (i.e., a liquid or a gas) is conducted, totransfer heat from one location to another. The fins are typicallyjoined to the pipes by friction, solder or epoxy. Discontinuousinterfaces between the fins and the associated pipe typically presentsignificant thermal resistance, and the efficiency of heat transfer iscompromised accordingly. Also, the effective contact area between a heatpipe and its surrounding fins is generally limited due to the process bywhich the fins, usually made of sheet metal, are formed. Moreover, thefins are often attached manually one at a time, making the assemblyprocedure labor intensive and costly.

[0007] It is also known that certain electronic components or devicesmay be encapsulated with a conductive plastics compound to form heatradiating surfaces about the devices. The geometry of the radiatingsurfaces is, however, limited by the flow characteristics of theplastics compound, its brittle nature when loaded with a material toincrease its thermal conductivity, and a generally lower radiatingefficiency in comparison to that obtained with most metals.

[0008] U.S. Pat. No. 5,040,589 (Aug. 20, 1991), discloses a method andapparatus for injection molding of metal alloys, wherein a selectedalloy is heated to a thixotropic or semi-solid state, and then injectedas a slurry into a mold to form a useful product. See also U.S. Pat.Nos. 4,694,881 and 4,694,882, both issued Sep. 22, 1987, and disclosingmethods for making thixotropic materials. All relative portions of thementioned '589, '881 and '882 U.S. patents are incorporated byreference. Certain metal products typically formed by die casting andsubsequent finishing steps may be produced instead by injection moldingof magnesium alloys, according to the patented methods. Such molding isclaimed to result in net-shape products with lower porosity, closerdimensional tolerances, and reduced manufacturing cost with respect tothe same products when die cast.

[0009] As far as is known, however, injection molding of metals has notbeen used to produce heat exchanging structures directly on thermallyconductive core objects or heat sources.

SUMMARY OF THE INVENTION

[0010] According to the invention, heat exchange surfaces are formed ona core object by placing at least a part of a thermally conductive coreobject within a mold cavity formed to define one or more heat exchangesurfaces, injecting a heated metal slurry into the mold under apredetermined pressure, and cooling the heated metal slurry thus forminga substantially continuous void free interface between the core objectand the metal slurry when hardened for effective heat transfer acrossthe interface.

[0011] For a better understanding of the invention, reference is made tothe following description taken in conjunction with the accompanyingdrawing and the appended claims.

BRIEF DESCRIPTION OF THE DRAWING

[0012] In the drawing:

[0013]FIG. 1 is a schematic representation of a heat exchange assemblyhaving a core pipe and associated heat exchanging fins, according to theinvention;

[0014]FIG. 2 is a perspective view of a heat exchange assembly similarto the assembly of FIG. 1;

[0015]FIG. 3 illustrates a molding process for producing a heat exchangeassembly according to the invention;

[0016]FIG. 4 is a scanning electron microscope (SEN) image of aninterface between a core pipe and associated heat exchanging fins,according to the invention;

[0017]FIG. 5 is a graph identifying relative amounts of metallicelements at both sides of the interface in FIG. 4;

[0018]FIG. 6 illustrates a cooling system for an electronics equipmentenclosure, according to the invention;

[0019]FIG. 7 shows a part of the cooling system of FIG. 6;

[0020]FIG. 8 is an assembly view of a heat sink arrangement for anelectronic component, according to the invention;

[0021]FIG. 9 is an assembly view of a baseboard heating system,according to the invention;

[0022]FIG. 10 illustrates an automotive radiator assembly, according tothe invention;

[0023]FIG. 11 illustrates an environmental cooling system, according tothe invention;

[0024]FIG. 12 is a perspective view of a half mold or die plate used inthe present method; and

[0025]FIG. 13 is a detail view of one end of the half mold shown in FIG.12.

DETAILED DESCRIPTION OF THE INVENTION

[0026]FIG. 1 represents a model of a heat exchange assembly 10,according to the invention. The assembly 10 includes a core pipe 12 madeof copper or equivalent material having relatively high thermalconductivity, and one or more heat exchanging fins 14 which are closelyjoined as a unit about the circumference of the core pipe 12. The fins14 are made of, for example, a magnesium alloy (e.g., type AZ91D) whichis capable of being heated to a thixotropic state, and then injected asa slurry into a mold cavity under pressure whereby the fins 14 areformed on the core pipe 12, as explained further below. The materialforming the fins 14 also has a relatively high thermal conductivity, forexample, about 42 BTU/ft·hr·deg F. for the mentioned type AZ91Dmagnesium alloy. The fins 14 are formed with a common cylindrical base16 whose inner circumference establishes a substantially continuous,void-free interface 18 with the outer circumference of the core pipe 12once the molded, heated slurry is allowed to cool and harden. Theinterface 18 between the inner circumference of the fin base 16 and theouter circumference of the core pipe 12 thus ensures an efficient heattransfer across the interface 18 in either direction.

[0027]FIG. 2 is a perspective view of a heat exchange assembly 20 whichwas constructed according to the model in FIG. 1. The assembly 20 has acopper core pipe 22 and a total of seven circular heat exchanging fins24. The pipe 22 has an outer diameter of about 0.375 inches, an innerdiameter of 0.300 inches, and an overall length of 6.0 inches. Each ofthe fins 24 has a diameter of about 2.0 inches, and extends radiallyfrom a common cylindrical base 26 whose outer diameter is about 0.500inches. The fins 24 are each about 0.040 inches thick, and are spacedapart from one another in the axial direction by about 0.375 inches. Thematerial used to form the heat exchanging fins 24 was a magnesium alloytype AZ91D. The alloy was initially heated to about 900 degrees F., andthen injected as a thixotropic slurry into a mold cavity in which thecopper core pipe 22 was previously placed and supported along an axis ofthe cavity. The injection pressure was approximately two tons perprojected square inch.

[0028]FIG. 3 depicts a process by which the heat exchange assembly 20 ofFIG. 2 and other heat exchange devices can be manufactured, according tothe invention. A series of die plates or half molds 32 are arranged intandem for linear movement about the perimeter of an injection moldingmachine 34. Another series of die plates or half molds 36 are arrangedin tandem for linear movement about the periphery of another injectionmolding machine 38, which may be substantially identical to the machine34. The die plates 32 and 36 may also be substantially identical to oneanother. See FIGS. 12 and 13. The molding machines 34, 38 are positionedso that corresponding ones of the die plates 32, 38 will face oneanother while being displaced by the machines 34, 38 along a commondirection of travel shown by arrows 40 in FIG. 3.

[0029] As seen in FIG. 12, each die plate 32, 36 forms a half-moldcavity 37 defining corresponding upper or lower halves of the heatexchanging fins 24 and common cylindrical base 26 of the assembly 20 inFIG. 2. A number of pairs of the die plates which face one another overa portion of the travel path 40, are urged by the associated machines34, 38 into a closed position thus forming full mold cavities withinthem. Guide pins 41 on either one of the confronting die plates 32, 38enter corresponding openings 43 formed in the other die plate, so thatthe confronting plates 32, 38 are properly aligned as they close againstone another. Inlets 39 that open at the back of each die plate 32, 36,communicate through a passage in the die plate with the half mold cavity37. As shown in FIG. 3, the inlets 39 of the die plates are positionedto align with corresponding chambers 47 in the machines 34, 38. A heatedthixotropic metal slurry is then discharged from the machine chambers 47into the die plate inlets 39 at a predetermined pressure and timeinterval.

[0030] Further, as shown in FIG. 13, axial ends of each of the dieplates 32, 36 have a semi-circular cutout 44 which is formed with raisedsemi-circular ribs 46 each having, e.g., a triangular cross section.Thus, when pairs of the die plates 32, 36 close with one another, a corepipe 42 (FIG. 3) can extend axially through the cutouts 44 in all theclosed pairs of the die plates 32, 36, with insubstantial leakage whenheated material is injected into the mold cavities 37 within the closedplates. That is, the raised ribs 45 create an interference fit betweenthe outer diameter of the core pipe 42 and the inner periphery of thecutouts 44 in each of the die plates 32, 36. The ribs 46 deform thesofter pipe 42 (or other core part) radially by, e.g., a few thousandthsof an inch, similar to compression fittings known in the plumbing,automotive and utility fields. Depending on the wall strength of thepipe 42, it may be necessary to insert a solid rod or mandrel 48 insidethe pipe, as shown in FIG. 3, in order to prevent deformation orcollapse of the pipe wall in response to the outside pressure of theinjected slurry.

[0031]FIG. 4 is a scanning electron microscope (SEM) image showing acontact interface 50 between a magnesium alloy base 52 that wasinjection molded over a surface of a copper pipe 54. Specifically, atype AZ91D magnesium alloy was injected at about 900 degrees F. into amold cavity containing the copper pipe 54, within about {fraction(1/10)}th of a second at a pressure of about two tons per projectedsquare inch. The image of FIG. 4 represents a 2,000 magnificationsetting for the SEM, and a distance of 10 um is shown by a scale line56. As seen in FIG. 4, interface 50 is substantially continuous andvoid-free.

[0032]FIG. 5 is a graphic representation showing relative amounts ofmetallic elements at both sides of the interface 50 in FIG. 4. Units ofdistance (arb) along the x-axis in FIG. 5 are such that about 2,200 arbunits equals 50 μm. A region 60 about the interface 50 wherein bothcopper and magnesium elements are detectable, extends over only about140 arb units or 3 μm. That is, the interface 50 is quite sharp.Relatively small counts of Mg and Cu appear at opposite sides of theinterface 50 because background was not subtracted in the graph of FIG.5.

[0033]FIG. 6 shows a cooling system 70 for an electronics equipmentenclosure 72, and FIG. 7 shows a part of the cooling system 70 in FIG.6. One or more heat conductive pipes 74 have a number of heat radiatingfins 76 molded over end portions of each of the pipes 74, according tothe present invention. Central portions of the pipes 74 intermediate theend portions form a 180 degree bend and are supported in thermalconducting relation within or in contact with a source of heat, forexample, a chassis, a power supply cabinet, or other heat-generatingelectrical equipment 78. A vapor barrier or environmental gasket 80 madeof, e.g., a soft elastomer or rubber material creates a water-tight sealbetween the heat pipes 74 and the equipment 78.

[0034] An air blower 82 disposed, e.g., at the bottom of the equipmentenclosure 72 directs an outside air flow 84 past the sets of radiatingfins 76 on each of the pipes 74. Accordingly, heat conducted by theintermediate portions of the pipes 74 away from the heat source 78, isdissipated via the radiating fins 76 and the air flow 84 to the outsideenvironment.

[0035]FIG. 8 is an exploded view of a heat sink device 90 for anelectronic component 92, e.g., a processor chip. A relatively thin metalsubframe 94 is fastened over one or more surfaces of the component 92,and placed with the component inside an injection mold cavity whichdefines a number of vertical heat dissipating elements in the form of,e.g., cylindrical rods 95. A thixotropic metal slurry is injected intothe cavity and adheres to the thin metal subframe 94 while filing voidsin the cavity corresponding to the rods 95. Perforations 96 in thesubframe 94 are also filled with the injected slurry to form mechanical“locks” between the slurry when cooled and hardened, and the subframe.The thin metal subframe 94 aids in protecting the component 92 from theelevated temperature of the slurry while in the mold cavity.

[0036] When the component with the subframe 94 and integral rods 95 areremoved from the mold cavity, the completed assembly may be mounted on,e.g., a printed wiring board via fasteners (not shown) that pass throughmounting holes 99 in side flanges 98 of the subframe 94, and the wiringboard. Contact pins or leads of the component 92 may then be soldered orotherwise connected to corresponding conductors associated with thewiring board.

[0037]FIG. 9 is an assembly view of a baseboard heating system 100. Thesystem 100 comprises a core heat conducting pipe 102 through which aheated working fluid (e.g., hot water) is circulated by an outside pump(not shown). A number of heat radiating fins 104 are formed with acommon cylindrical base 106 on the outer circumference of the fluid pipe102, by way of an injection molding process such as that described inconnection with FIG. 3. Various length sections of the fluid pipe 102with the molded radiating fins 104 may be produced initially, and thenconnected to one another through straight or angled pipe couplings tofit a particular application. Once in place, a slotted protective cover108 is fastened over the pipe 102 and the associated fins 104.

[0038]FIG. 10 shows an automotive radiator assembly 120. A number ofheat conductive (e.g., copper) metal core pipes are arranged paralleland co-planar with one another, after a series of heat radiating fins124 are molded with a common cylindrical base 126 over each of the pipes122 per the present method. Opposite open ends of the pipes 122 arejoined in fluid communication with corresponding header or end pipes128, 130. When heated engine coolant is pumped through one of the endpipes 128, 130, the coolant is directed through each of the core pipes122 and cooled by outside air which has been directed to flow over theradiating fins 124 on the pipes 122. The coolant is then returnedthrough the opposite end pipe to be pumped and circulated through anassociated engine.

[0039]FIG. 11 illustrates an environmental cooling system 150. One ormore sections of a heat conducting, metal core pipe 152 have a series ofheat exchanging fins 154 with a common cylindrical base 156 molded overthe outer circumference of the pipe 152, according to the presentmethod. A cooled working fluid such as, for example, an evaporatedrefrigerant, water or air is directed under pressure through an inlet158 of the pipe 152. Warm air to be cooled is directed by outside means(e.g., a blower or fan) between the fins 154 so that the fins absorbheat and conduct it through the fin base 156 and the pipe 152 into theworking fluid. The heated working fluid exits from an outlet 160 of thecore pipe 152, and cooled air 162 is available to be channeled wheredesired by suitable means.

[0040] The various heat exchanging apparatus disclosed herein are highlyefficient because of the formation of a substantially continuous,void-free thermal interface between a thermally conductive core pipe ortube, and a number of heat exchanging fins which are injection moldedunder pressure over the pipe rather than being formed and attachedindividually. The present injection molding process may also yield finshaving thinner cross-sections and less weight than conventional fins.Magnesium and aluminum alloys are highly thermally conductive materialshaving high strength-to-weight ratios, and are both well suited forinjection molding into the form of heat radiating or cooling finsaccording to the present process.

[0041] Importantly, the present process yields an increased contact areabetween a number of heat exchanging fins and their associated-core pipeor component when compared to prior configurations using individualfins. The process can be used to form heat sink configurations forvarious electronic devices and products that must operate with adequatecooling, including large scale installations such as wireless telephonebase stations where heat generated by a number of active radiotransceivers within a confined space must be dissipated in an effectiveand efficient manner.

[0042] While the forgoing description represents preferred embodimentsof the invention, it will be obvious to those skilled in the art thatvarious changes and modifications may be made without departing from thespirit and scope of the invention pointed out by the following claims.

We claim:
 1. A method of forming heat exchange surfaces on a coreobject, comprising: placing at least a part of a thermally conductivecore object within a mold cavity that is formed to define one or moreheat exchange surfaces; injecting a heated metal slurry into the moldcavity under a predetermined pressure; and cooling the heated metalslurry thus forming a substantially continuous void free interfacebetween the core object and the metal slurry when hardened for effectiveheat transfer across the interface.
 2. A method according to claim 1,including heating a metal to a thixotropic state, and then performingsaid injecting step using the heated thixotropic metal as said metalslurry.
 3. A method according to claim 2, including raising thetemperature of the metal to about 900 degrees F. prior to said injectingstep.
 4. A method according to claim 2, including using type AZ91Dmagnesium alloy as said metal, and raising the temperature of said alloyto about 900 degrees F. prior to said injecting step.
 5. A methodaccording to claim 1, including forming the mold cavity to define one ormore fins about the core object.
 6. A method according to claim 1,including providing a heat conductive pipe as said core object.
 7. Amethod according to claim 6, including inserting a rigid rod axiallythrough the pipe thus avoiding deforming of the pipe during theinjecting step.
 8. A method according to claim 7, including forming themold cavity to define one or more fins as the heat exchange surfacesabout the outer circumference of the pipe.
 9. A method of forming heatexchange surfaces on a core object, comprising: arranging a first seriesof die plates in tandem for linear movement about a first perimeter of afirst molding apparatus; arranging a second series of die plates intandem for linear movement about a second perimeter of a second moldingapparatus; forming each of the first series of die plates to definefirst parts of one or more heat exchange surfaces; forming each of thesecond series of die plates to define corresponding second parts of oneor more of said heat exchange surfaces; positioning the first and thesecond molding apparatus so that corresponding ones of the first and thesecond die plates face one another while being displaced by theapparatus along an axial direction with respect to an elongatedthermally conductive core object; placing the core object between thefacing ones of the first and the second series of die plates; urging thefacing die plates to a closed position thus forming full mold cavitiescorresponding to the heat exchange surfaces about the core object;injecting a heated metal slurry into the full mold cavities under apredetermined pressure; and cooling the heated metal slurry thus forminga substantially continuous void free interface between the core objectand the metal slurry when hardened for effective heat transfer acrossthe interface.
 10. A method according to claim 9, including heating ametal to a thixotropic state, and then performing said injecting stepusing the heated thixotropic metal as said metal slurry.
 11. A methodaccording to claim 10, including raising the temperature of the metal toabout 900 degrees F. prior to said injecting step.
 12. A methodaccording to claim 10, including using type AZ91D magnesium alloy assaid metal, and raising the temperature of said alloy to about 900degrees F. prior to the injecting step.
 13. A method according to claim9, including forming the die plates to define one or more fins about thecore object.
 14. A method according to claim 9, including providing aheat conductive pipe as said elongated core object.
 15. A methodaccording to claim 14, including inserting a rigid rod axially throughthe pipe, thus avoiding deforming of the pipe during the injecting step.16. A method according to claim 15, including forming the die plates todefine one or more fins as said heat exchange surfaces about the outercircumference of the pipe.
 17. A heat exchanging device producedaccording to the method of claim
 1. 18. A heat exchanging deviceproduced according to the method of claim 9.